The present invention relates to a semiconductor device, an integrated circuit including the semiconductor device, a control IC including the semiconductor device for controlling a switching power supply, and the switching power supply.
A control IC for controlling a switching power supply (hereinafter referred to simply as the “control IC”) is an exclusive IC for controlling individual switching transistors exhibiting a high breakdown voltage. The control IC obtains the power supply thereof by making the switching transistors work in the operating state thereof. However, it is necessary to feed a starting current from a starter circuit to the control IC at the start thereof. Usually, the starter circuit is incorporated into a semiconductor substrate, into which the control IC is incorporated, to decrease the component parts and to simplify the power supply system.
The starting current is obtained by rectifying an AC input signal of 100 to 240 V. In order to feed the starting current obtained as described above to the starter circuit, it is necessary for a device of normally-on-type (hereinafter referred to as the “normally-on device”) disposed in the front stage of the starter circuit to exhibit a breakdown voltage of 450 V or higher. For integrating the normally-on device and the control IC monolithically into a unit, the normally-on device is implemented in the form of a lateral junction-type field effect transistor exhibiting a high breakdown voltage. Hereinafter the junction-type field effect transistor will be referred to as a “JFET”. The current driving capability of the JFET determines the design specifications, based on which a switching power supply is designed.
FIG. 11 and FIG. 12 are block circuit diagrams of conventional switching power supplies. The configuration shown in FIG. 11 rectifies and makes an AC input smooth and feeds the rectified and smooth input to input terminal 232, the breakdown voltage thereof is high, (hereinafter referred to as “VH terminal 232”) of control IC 231. The configuration shown in FIG. 12 conducts half-wave rectification of an AC input and feeds the rectified input to VH terminal 232.
As shown in FIG. 11 or 12, the switching power supply conducts full-wave rectification of the AC input, supplied commercially for example, with rectifier 202 and charges power supply capacitor 203 with the DC voltage obtained by the full-wave rectification. The switching power supply makes control IC 231 control the ON and OFF of MOSFET 219, working as a switching device and connected to primary coil 206 of transformer 205, to induce a voltage based on the voltage of power supply capacitor 203 across secondary coil 208 of transformer 205. The switching power supply rectifies and makes the induced voltage smooth and feeds the rectified and smooth voltage to a load (not shown).
As the plug of the switching power supply is pulled out from a receptacle and the power supply from the AC input is interrupted, the primary-side input voltage lowers. If the switching power supply keeps operating in this state, the ON-period of MOSFET 219 is prolonged, causing heat generation in MOSFET 219. To prevent this problem from occurring, the switching power supply is provided with a brownout function that stops the switching operation of the power supply when the input voltage lowers.
In order to realize the brownout function in the conventional switching power supply shown in FIG. 11 or 12, brownout input terminal (hereinafter referred to as “BO terminal”) 262 that detects the primary-side voltage of the power supply is disposed on control IC 231. BO terminal 262 and is connected to the intermediate node of a series resistance circuit consisting of two resistors 251 and 252 connected in parallel to power supply capacitor 203.
The primary-side input voltage is divided by resistors 251 and 252, input to brownout comparator (hereinafter referred to as “BO comparator”) 244 via BO terminal 262 and compared with a predetermined voltage in BO comparator 244. When the input voltage from BO terminal 262 is lower than the predetermined voltage, the brownout function works to stop the switching operation of MOSFET 219 driven by driver circuit 246.
FIG. 13 is a circuit diagram of a starter circuit used in the conventional switching power supply. As shown in FIG. 13, conventional starter circuit 241 includes input terminal 261 exhibiting a high breakdown voltage (hereinafter referred to as “VH terminal 261”), on/off signal input terminal (hereinafter referred to as “on/off terminal”) 263, and power supply voltage terminal (hereinafter referred to as “VCC terminal”) 264. Starting device 265 in starter circuit 241 includes first JFET 281 that makes a current flow to VCC terminal (power supply voltage terminal) 235 of control IC 231 via VCC terminal 264 at the start of the power supply and second JFET 282 that keeps NMOS transistor 268 disposed on the current path of first JFET 281 in the ON-state.
In addition to realizing the brownout function, the input voltage from an external wiring is sometimes divided by voltage dividing resistors before the input thereof to a control IC. It is necessary for the voltage dividing resistor connected to the external wiring to exhibit a high breakdown voltage when the external wiring voltage is high. For suppressing the electric power consumed all the time, it is necessary for the voltage dividing resistors to exhibit high electrical resistance. Therefore, a voltage dividing system that uses externally added resistors is usually employed. For example, so-called power-factor-improving circuit 1200 for suppressing higher harmonic currents is sometimes disposed between rectifier 202 and power supply capacitor 203 in FIG. 12.
FIG. 14 is a block circuit diagram of a conventional power-factor-improving circuit. The circuit configured as shown in FIG. 14 charges capacitor 1008 with the DC voltage obtained by rectifying and smoothing an AC input signal (100-240 V). The DC voltage of capacitor 1008 that is high is lowered by voltage-dividing resistance circuit 1001 to be low enough to be inputted to control IC 1100 for improving the power factor (hereinafter referred to as a “power-factor-improving IC”) and the lowered DC voltage is inputted to input terminal 1101 of power-factor-improving IC 1100. In the same manner, the DC voltage of capacitor 203 that is high is lowered by voltage-dividing resistance circuit 1009 to be low enough to be inputted to power-factor-improving IC 1100 and the lowered DC voltage is inputted to feedback terminal 1102 of power-factor-improving IC 1100. Power-factor-improving IC 1100 generates, based on the signals inputted to input and feedback terminals 1101 and 1102 thereof, a pulse-width-control signal that adjusts the AC current waveform to be similar to the AC voltage waveform. Switching transistor 1005 makes an intermittent current flow through boost inductor 1003. The intermittent current is outputted with the waveform thereof changed to be sinusoidal with rectifier 1007 and capacitor 203. Resistor 1002 is disposed to feed a power supply to power-factor-improving IC 1100. Resistor 1004 is disposed to adjust the gate current of switching transistor 1005. Usually, the resistors described above, the resistors in input-side voltage-dividing-resistance circuit 1001 and the resistors in output-side voltage-dividing-resistance circuit 1009 are added externally.
A switching power supply, which improves the overcurrent detection accuracy in the state, in which an input voltage variation is caused or the input voltage range is changed over, by a configuration that changes the reference signal fed to an overcurrent detecting comparator in response to the variation of the output voltage from an input voltage detector circuit, has been known to persons skilled in the art (See, for example, Unexamined Japanese Patent Application Publication No. 2005-94835). A semiconductor device, which includes a semiconductor substrate, a circular diffusion layer, the potential thereof is floating, on a major surface of the semiconductor substrate, and a resistance layer above the circular diffusion layer with an insulator layer interposed between the resistance layer and the diffusion layer, has been known to persons skilled in the art (See, for example, Unexamined Japanese Patent Application Publication No. 2000-252426). A semiconductor device for the switching regulator, which incorporates therein a starting resistor exhibiting a high breakdown voltage and high resistance, has been known to persons skilled in the art (See, for example, Unexamined Japanese Patent Application Publication No. 2001-313367, Counterpart U.S. Pat. No. 6,492,689).
Since it is necessary to externally add a very resistive element, exhibiting a high breakdown voltage and high electrical resistance (hereinafter referred to simply as a “very resistive element”), to the control IC in the conventional switching power supply described above, the component parts costs and assembly costs thereof increase substantially and the conventional switching power supply is prevented from being reduced in size. Although a very resistive element is necessary for detecting the input voltage in the switching power supply disclosed in Unexamined Japanese Patent Application Publication No. 2005-94835, any concrete configuration is not described thereon. Although a resistance layer working for a starting resistor is formed on the field limiting ring surrounding the active region of the semiconductor chip in the semiconductor device disclosed in the Unexamined Japanese Patent Application Publication No. 2000-252426 and U.S. Pat. No. 6,492,689, the resistance layer is formed not for realizing a brownout function.
In view of the foregoing, it would be desirable to obviate the problems described above. It would be also desirable to provide a switching power supply including a very resistive element, connected to a high voltage that facilitates reducing the component parts costs, the assembly costs and the size thereof. It would be further desirable to provide a semiconductor device necessary for realizing the desirable switching power supply as described above, a semiconductor integrated circuit including the semiconductor device described above and a control IC for controlling the switching power supply.